FAA Order 8700.1, Volume 2, Chapter 1, page 1-46 and 1-47, paragraph 9.B. states:
“B. Logging Time. Unless the vehicle is type certificated as an aircraft in a category listed in FAR § 61.5(b)(1) or as an experimental aircraft, or otherwise holds an airworthiness certificate, flight time acquired in such a vehicle may not be used to meet requirements of FAR Part 61 for a certificate or rating or to meet the recency of experience requirements.”
In order for the flight time to be logable, the flight time must have been acquired in an aircraft that is identified as an aircraft category as listed in § 61.5(b)(1). This means that it fits into one of the four following categories:
(1) An aircraft of U.S. registry that has a civilian type designation and has a current standard, limited, or primary airworthiness certificate;
(2) An aircraft of U.S. registry that has a civilian type designation and has a current airworthiness certificate other than standard, limited, or primary;
(3) An aircraft of foreign registry that has a civilian type designation and is properly certificated by the country of registry; or
(4) A military aircraft under the direct operational control of an armed force of the United States.
Can you log flight time in an ultralight? Yes, you can log it in a hang glider,for that matter. It cannot be used toward any certificate, rating operating privilege, or recency of flight experience necessary for any airman certificate...but you can log it.
Logging ultralight time toward "building time" has the pronounced effect of making the person doing the logging look stupid. If that person attempts to attend an interview some day with ultralight time in the logbook, it will very well likely cost that person the interview right out of the gate.
One could push it and start logging time under canopy when skydiving or parachuting (or base jumping). Without arguement, a higher performance eliptical canopy takes quick reaction, good situational awareness, and a high degree of skill to fly and land. However, attempting to put this in a logbook applicable to an airman certificate is not only foolish, but looks foolish.
One can also log time spent watching aircraft fly overhead, hours in bed with a good woman, and numbers of flies killed on any given summer day while mucking out stalls in a stable. I wouldn't recommend it, however.